Among light emissions from sun, fluorescent lamps, Braun tubes and the like, UV rays in the short wavelength region of up to 400 nm have many detrimental effects, not only to the human body including sunburn, speckles, carcinogenesis and eyesight failure, but also to articles, including mechanical strength degradation, outer appearance degradation (e.g., fading), and discoloration of printed matter.
To address such problems, especially the problem that the UV transmitted by windows into buildings or automobiles causes degradation of interiors, fading of fabrics, and sunburn of the human body, it is required to control transmission of UV by windows. In such UV screening applications, there are used film, glass, and plastic articles which are provided with a UV-screening function by incorporating UV absorbers therein or by applying a coating solution containing a UV absorber onto a substrate to form a UV-screening film thereon. For the existing windows and substrates, it is a common practice to provide them with a UV-screening function by applying a coating solution containing a UV absorber thereto to form a UV-screening film.
UV absorbers used in the prior art include salicylic acid, benzophenone, benzotriazole and cyanoacrylate compounds. Since these conventional UV absorbers cannot form a coating by themselves, they are typically used as additives to binder components. Also, these UV absorbers will evaporate or volatilize off over a long period of service, resulting in substrates having a degraded UV-screening function. In order that coated substrates retain a UV-screening function for a long time, the UV absorbers must be used in larger amounts, which give rise to problems such as bleed-out of UV absorber onto the surface and clouding of the substrate.
It was thus proposed to incorporate silicon into UV absorbers for anchoring. Such approaches are successful to some extent. In order to incorporate UV absorbers into silicone resins with good heat resistance and light resistance, many attempts were made to react benzophenone UV absorbers with alkoxysilanes.
For instance, JP 4092522 discloses a curable UV absorber obtained from reaction of hydroxybenzophenone with an epoxy-containing alkoxysilane in the presence of an ammonium salt catalyst. JP-A 2000-160130 discloses a curable UV absorber obtained from reaction of tetrahydroxybenzophenone with an isocyanato-containing alkoxysilane in the presence of a tin catalyst. In these patents, links are formed in the structure of Ph-O—CH2—CH(OH)— and Ph-O—C(O)—NH—, respectively. In both cases, the final reaction products are highly hydrophilic and susceptible to hydrolysis, and form UV-absorbing films on the substrate surface which are insufficient in durable adhesion under humid atmosphere. In addition, the catalyst can be left in the final reaction product, exerting detrimental effects on shelf stability.
JP-A H07-278525 and JP 3648280 disclose a curable UV absorber having only one alkoxysilane obtained by converting hydroxybenzophenone into allyl ether form and reacting it with hydrosilane, and a UV-absorbing film comprising the same. Since the hydroxybenzophenone alkyl ether alkoxysilane is an alkoxysilane having a very large substituent, it has the drawback that it is rather unsusceptible to hydrolysis, ineffective to form a film by itself, and less adhesive.
JP-A S57-21390 discloses a UV absorber having an alkoxysilyl group incorporated into a benzophenone skeleton via an amide bond. However, a composition comprising the same is less adhesive to substrates. JP-A S57-21476 discloses a UV absorber encompassing the benzophenone derivative (I-a) according to the present invention. However, the UV absorber produced by the disclosed method has the serious problem that it tends to condensate into a polymer or gel. JP-A S58-213075 discloses a UV absorber which is similar to that of JP-A S57-21476, wherein the alkoxy group bonded to silicon is changed so as to prevent the compound from gelation. Undesirably the production process is complex.